Abstract:

A method of friction stir welding and a non-consumable multi-section faced
retractable shoulderless variable penetration friction stir welding tool.
The tool includes a substantially cylindrical body portion, a head
portion, and a tip section, each integral to the tool. The body portion
has a longitudinal axis about which it is rotable, a diameter, a sidewall
substantially parallel to the longitudinal axis, a proximal end, and a
distal end. The head portion is located at the distal end of the body
portion. The head portion includes a multi-section face, having a first
face section and a second face section, that converges to the tip
section. The tool is retractable, reduces overheating, improves weld
quality by reducing internal voids and lack of fusion, and facilitates
variable penetration welds.

Claims:

1. A non-consumable multi-section faced retractable shoulderless variable
penetration friction stir welding tool for use in joining a first
workpiece, having a first thickness, and a second workpiece, having a
second thickness, by friction stir welding, comprising:a substantially
cylindrical body portion, a head portion, and a tip section, each
integral to the tool, the body portion having a longitudinal axis about
which it is rotable, a diameter, a sidewall substantially parallel to the
longitudinal axis, a proximal end, and a distal end; andthe head portion
located at the distal end of the body portion having a base with a
diameter substantially equal to the diameter of the body portion forming
a transition between the body portion and the head portion, and the head
portion having a multi-section face with a first face section nearest the
body portion and a second face section nearest the tip section wherein
the multi-section face converges to the tip section having a diameter and
a center wherein the center is located substantially on the longitudinal
axis, a height from the distal-most portion of the tip section to the
base along the longitudinal axis and the first face section having a
first section horizontal projection component with a magnitude of less
than fifteen percent of the body portion diameter and the first face
section converges at a first face opening angle to the second face
section thereby forming a substantially frustoconical shape.

2. The tool of claim 1, wherein the second face section forms a
substantially frustoconical shape with the second face section converging
to the tip section at a second face opening angle between approximately
70 degrees and approximately 160 degrees.

3. The tool of claim 1, wherein the first face opening angle is between
approximately 40 degrees and approximately 120 degrees.

4. The tool of claim 1, wherein the first section horizontal projection
component has a magnitude of less than eight percent of the body portion
diameter.

5. The tool of claim 1, wherein the first face section is curved having a
radius of curvature of less than fifteen percent of the body portion
diameter.

6. The tool of claim 1, wherein the portion of the rotating non-consumable
multi-section faced shoulderless variable penetration friction stir
welding tool that is plunged into the joint is a portion of the head
portion such that a portion of the first face section is within the first
and second workpieces and a portion of the first face section is outside
the first and second workpieces.

7. A non-consumable multi-section faced retractable shoulderless variable
penetration friction stir welding tool for use in joining a first
workpiece, having a first thickness, and a second workpiece, having a
second thickness, by friction stir welding, comprising:a substantially
cylindrical body portion, a head portion, and a tip section, each
integral to the tool, the body portion having a longitudinal axis about
which it is rotable, a diameter, a sidewall substantially parallel to the
longitudinal axis, a proximal end, and a distal end; andthe head portion
located at the distal end of the body portion having a base with a
diameter substantially equal to the diameter of the body portion forming
a transition between the body portion and the head portion, and the head
portion having a multi-section face with a first face section nearest the
body portion and a second face section nearest the tip section wherein
the multi-section face converges to the tip section having a diameter and
a center wherein the center is located substantially on the longitudinal
axis, a height from the distal-most portion of the tip section to the
base along the longitudinal axis and the first face section having a
first section horizontal projection component with a magnitude of less
than approximately eight percent of the body portion diameter.

8. The tool of claim 7, wherein the second face section forms a
substantially frustoconical shape with the second face section converging
to the tip section at a second face opening angle between approximately
70 degrees and approximately 160 degrees.

9. The tool of claim 7, wherein the first face section forms a
substantially frustoconical shape with the first face section converging
to the second face section at a first face opening angle.

10. The tool of claim 9, wherein the first face opening angle is between
approximately 40 degrees and approximately 120 degrees.

11. The tool of claim 7, wherein the first face section is substantially
orthogonal to the longitudinal axis.

12. The tool of claim 11, wherein the first section horizontal projection
component has a magnitude of less than approximately five percent of the
body portion diameter.

13. The tool of claim 7, wherein the first face section is curved.

14. The tool of claim 7, wherein the portion of the rotating
non-consumable multi-section faced shoulderless variable penetration
friction stir welding tool that is plunged into the joint is a portion of
the head portion such that a portion of the first face section is within
the first and second workpieces and a portion of the first face section
is outside the first and second workpieces.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application is a division and claims the benefits of the
previously filed and currently pending U.S. patent application given Ser.
No. 11/109,519, filed on Apr. 19, 2005, which is a continuation-in-part
and claims the benefits of the previously filed U.S. patent application
given Ser. No. 10/970,907, filed on Oct. 22, 2004, now U.S. Pat. No.
7,234,626, all of which are incorporated by reference as if completely
written herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002]This invention was not made as part of a federally sponsored
research or development project.

TECHNICAL FIELD

[0003]The present invention relates to the field of friction stir welding;
particularly, to a single piece non-consumable friction stir welding tool
and methods that can perform variable penetration welds, variable width
welds, weld workpieces of differing thicknesses, weld workpieces having
complex curvature, retract from the weld during welding without producing
an exit hole, and improve the quality of friction stir welds.

BACKGROUND OF THE INVENTION

[0004]Those in the wide ranging materials joining industries have
recognized the benefits of friction stir welding (FSW) since its
invention, only to be precluded from widespread application due to a
number of factors. FSW is a relatively simple method of solid phase
welding developed by The Welding Institute in the early 1990's. The
conventional process utilizes a specially shaped nonconsumable
cylindrical tool with a profiled pin, often threaded, extending from a
shoulder of the tool, that is rotated and plunged into a joint formed by
abutting edges of the workpieces that are to be joined until a surface of
the shoulder contacts the surface of the workpieces. The rotating tool
plasticizes a region of the workpieces around the pin and beneath the
shoulder. The tool is then advanced along the joint. The rotation of the
tool develops frictional heating of the workpieces, from both shoulder
friction and pin friction, as well as adiabatic heating, and the tool
forces plasticized workpiece material from the leading edge of the tool
to the rear of the tool where it consolidates and cools to form a high
quality weld.

[0005]The FSW tool is generally a cylindrical piece with a shoulder face
that meets a pin that projects from the shoulder face at a right angle,
as illustrated in U.S. Pat. Nos. 5,460,317 and 6,029,879. In some
instances, the pin actually moves in a perpendicular direction in an
aperture formed in the face of the shoulder, as illustrated in U.S. Pat.
Nos. 5,611,469; 5,697,544; and 6,053,391. The face of the shoulder may be
formed with an upward dome that is perpendicular to the pin, as
illustrated in U.S. Pat. Nos. 5,611,479; 5,697,544; and 6,053,391. The
dome region and an unobstructed shoulder face to pin interface have been
considered essential for the proper frictional heating of the workpiece
material. Traditional thinking held that the dome region of the shoulder
serves to constrain plasticized material for consolidation at the
trailing edge of the FSW tool so as to prevent it from extruding out from
under the sides of the tool. For example, U.S. Pat. No. 5,813,592 states
at column 1, lines 42-51, that "In order to achieve a proper
consolidation of the weld metal the probe bottom part (shoulder) must
maintain during the whole welding operation (forward movement) in an
intimate contact with [the] surface of the joined members. If the probe
shoulder during this forward movement even temporarily `lifts` from the
surface a small amount of plasticised welding material will be expelled
behind the probe thus causing occurrence of voids in the weld since there
is no available material to fill the vacant space after the expelled
material." The present invention proves this long-held belief false.

[0006]Since FSW is a solid-state process, meaning there is no melting of
the materials, many of the problems associated with other fusion welding
methods are avoided, including solidification cracking, shrinkage, and
weld pool positioning and control. Additionally, FSW minimizes distortion
and residual stresses. Further, since filler materials are not used in
FSW, issues associated with chemical segregation are avoided. Still
further, FSW has enabled the welding of a wide range of alloys that were
previously unweldable. Yet another advantage of FSW is that it does not
have many of the hazards associated with other welding means such as
welding fumes, radiation, high voltage, liquid metals, or arcing.
Additionally, FSW generally has only three process variables to control
(rotation speed, travel speed, and pressure), whereas fusion welding
often has at least twice the number of process variables (purge gas,
voltage, amperage, wire feed speed, travel speed, shield gas, and arc
gap, just to name a few). Perhaps most importantly, the crushing,
stirring, and forging of the plasticized material by the FSW tool often
produces a weld that is more reliable than conventional welds and
maintains material properties more closely to those of the workpiece
properties, often resulting in twice the fatigue resistance found in
fusion welds.

[0007]Despite all the advantages of FSW, it has only found very limited
commercial application to date due to many difficulties associated
therewith. One early problem associated with single-piece FSW tools 90,
as seen in FIG. 1, was that they leave an exit hole 80 in the weld 40, as
seen in FIG. 5, that must be filled after completion of the friction stir
weld. Such single-piece FSW tools 90 are also plagued with premature
breakage of the pin 92 during welding, resulting in the pin 92 being
permanently lodged in the weld 40. Such breakage is often attributed to
tool design that has relatively poor heat distribution and areas of high
stress concentration, such as at the pin 92 to shoulder 91 interface,
also known as the transition region 93, seen in FIG. 1. In an effort to
eliminate exit holes 82 the retractable pin tool 95 was developed, as
seen in FIG. 2. The retractable pin tool 95 essentially splits the
conventional shouldered FSW tool 90 into two separate components, namely
a shoulder portion 96 that is hollow and receives the pin 97 that may
extend and retract from the shoulder 96. The independent movement of the
pin 97 permits the pin 97 to be gradually withdrawn from the weld 40
while the shoulder 96 remains in contact with the workpieces 10, 20,
thereby eliminating the exit hole 80.

[0008]While the retractable pin tool 95 may eliminate the exit hole 82, it
has several drawbacks. The retractable pin tool 95 is prone to breakage
due to the high stress concentrations at the shoulder 96 to pin 97
interface. The retractable pin tool 95 is also susceptible to binding
between the pin 97 and the shoulder 96 as stirred weld metal can be
forced into the gap between the pin 97 and the shoulder 96.

[0009]Another problem with both conventional shouldered FSW tools 90 and
retractable pin tools 95 is the overheating caused by the shoulder 91,
96. During FSW with conventional shouldered FSW tools 90, 95 the weld 40
is repeatedly subjected to the pressure and rotation of the tool shoulder
91, 96. As a conventional FSW tool 90, 95 traverses a joint 35 the
material is first exposed to the leading edge of the shoulder 91, 96 that
is generally exerting a downward force on the workpieces 10, 20 of
several hundred pounds, often several thousand pounds, and is rotating at
RPM's ranging from under 100 rpm to over 1000 rpm, while traversing the
joint 35 rather slowly, generally less than ten inches per minute (IPM),
depending on the materials being joined and their thickness. Taking for
example a simple illustrative case of a conventional tool 90, 95
traversing a joint 35 at 6 IPM and 800 RPM, it takes 10 seconds to
traverse a one inch section of the joint 35 during which 80 revolutions
of the tool 90, 95 are made, resulting in 160 exposures of weld 40 to the
shoulder 91, 96 (an exposure at the leading edge and the trailing edge
for each revolution). Such repeated exposure to the shoulder 91, 96
results in the overheating of the weld 40 and the associated drawbacks.
Prior methods and apparatus have indicated that such top surface friction
heating and weld material containment contributed by the shoulder were
essential to FSW. In fact, the definition of friction stir welding in
most welding references includes the mention of a tool having a pin and a
shoulder, thus a tool lacking a shoulder, or a shoulderless tool, as in
the present invention, is a completely new concept.

[0010]Further, conventional shouldered FSW tools 90 and retractable pin
tools 95 are generally ineffective at joining workpieces 10, 20 of
different thickness, as seen in FIG. 6. This is due in large part to the
fact that such tools 90, 95 are designed for a specific pin 92, 97 length
for a particular material thickness. Such designs necessitate a unique
tool for each thickness of material to be joined. The retractable pin
tool 95 may reduce the number of tools needed to make welds in materials
having differing thicknesses, but it too is limited in that each
retractable pin tool 95 has a limited useful range established by the
diameter of the shoulder. For instance, if the material is too thick or
thin then under-heating or over-heating will occur. Additionally, one can
easily appreciate that the pin 97 of a retractable pin tool 95 designed
for use in joining 1/8'' thick sheets will be ineffective and will fail
if it is simply further extended from the shoulder 96 in trying to join
1/2'' thick plates.

[0011]Additionally, conventional shouldered FSW tools 90 and retractable
pin tools 95 cannot be used in joining workpieces having more than slight
curvature. Such tools 90, 95 provide inadequate contact, also referred to
as lift-off, or result in gouging of the workpieces, as seen in FIG. 18.
Such lift-off and gouging results in welds having reduced aesthetic
qualities that often require grinding of the surface and diminish the
mechanical properties of the weld.

[0012]Yet another problem associated with conventional shouldered FSW
tools 90 and retractable pin tools 95 is the flow characteristics
imparted on the weld material due to the transition region 93, labeled in
FIG. 1, between the shoulder 91 and the pin 92. The transition region 93
in shouldered tools 90, 95 often causes dead zones and eddies in the
material flow resulting in subsurface voids and lack of fusion in the
weld 40. Such problems greatly limit the robustness of the conventional
tools and methods, particularly on joints that vary in geometry or heat
distribution due to part shape or tooling.

[0013]A friction stir weld 40 created with conventional shouldered FSW
tools 90, 95 has several distinct regions, as seen in FIG. 3, where the
direction of travel of the tool 90 is into the paper. First, the metal
away from the immediate vicinity of the weld 40 that is not affected by
the weld is known as the base metal 50. Closer to the actual weld 40 is
the heat affected zone (HAZ) 60 where the material has experienced a
thermal cycle that has modified the microstructure and/or mechanical
properties, yet has no plastic deformation. Next, closer to the tool 90,
95 is the thermomechanically affected zone (TMAZ) 70 where the material
has seen limited plastic deformation by the tool 90, 95, and the heat
from the process has also exerted some influence on the material. With
the exception of aluminum, most materials exhibit recrystallization
throughout the TMAZ 70. Aluminum often exhibits recrystallization in only
a portion of the TMAZ, often referred to as the nugget. Within the TMAZ
70 is the stir zone 75, seen in FIG. 4, having non-uniform grain
structure from the violent deformation that materials in this region
undergo while hot. The stir zone 75 has a shoulder region 76 and a pin
region 77. The pin region 77 is that region that has been directly
exposed to the pin 92, whereas the shoulder region 76 is the region just
outside of the pin region 77 and below the shoulder 91, 96 of the tool
90, 95. The shoulder region 76 flares out further away from the pin 92,
97 near the surface of the workpiece nearest the shoulder 91, 96, due to
the effects of the shoulder 91, 96. This flared-out portion of the
shoulder region 76, or re-stir area, near the surface of the weld 40 is
the area most commonly exposed to overheating and the associated
annealing and overageing effects that reduce the weld properties.

[0014]Additionally, the design of conventional shouldered FSW tools 90, 95
is prone to excessive wear and poor heat and load distribution. These
problems are largely attributable to the longstanding belief that FSW
tools must have a relatively narrow pin and wide shoulder.

[0015]Accordingly, the art has needed a tool, and associated methods, that
eliminate the need for a shoulder and thereby eliminate the multitude of
problems associated with the shoulder. An ideal tool would be simple in
design and construction; inexpensive; allow for retractability during
welding thereby eliminating the exit hole; accommodate joining materials
of differing thicknesses; facilitate variable penetration depth; improve
weld quality by reducing internal voids and lack of fusion; and eliminate
the re-stir area of the stir region. While some of the prior art devices
attempted to improve the state of the art, none has achieved the unique
and novel configurations and capabilities of the present invention. With
these capabilities taken into consideration, the instant invention
addresses many of the shortcomings of the prior art and offers
significant benefits heretofore unavailable. Further, none of the above
inventions and patents, taken either singly or in combination, is seen to
describe the instant invention as claimed.

SUMMARY OF INVENTION

[0016]In its most general configuration, the present invention advances
the state of the art with a variety of new capabilities and overcomes
many of the shortcomings of prior methods in new and novel ways. In its
most general sense, the present invention overcomes the shortcomings and
limitations of the prior art in any of a number of generally effective
configurations.

[0017]In one of the many preferable configurations, the non-consumable
multi-section faced retractable shoulderless variable penetration
friction stir welding tool includes a substantially cylindrical body
portion, a head portion, and a tip section, each integral to the tool.
The body portion has a longitudinal axis about which it is rotable, a
diameter, a sidewall substantially parallel to the longitudinal axis, a
proximal end, and a distal end.

[0018]The head portion has a base with a diameter substantially equal to
the diameter of the body portion, thereby forming a transition between
the body portion and the head portion. The head portion includes a
multi-section face that converges to the tip section. The multi-section
face includes at least a first face section and a section face section.
The tool lack of a shoulder in the traditional sense of friction stir
welding therefore has numerous advantages that have long been overlooked
by those in the FSW industry. Prior methods and apparatus have indicated
that top surface friction heating and weld material containment were
essential to FSW.

[0019]The present invention's elimination, or minimization, of any portion
of the tool that contacts the top surface of either workpiece away from
the point at which the tool enters the workpiece(s) has several
advantages. One such advantage is the elimination of the primary source
of overheating. Additionally, another advantage of the present tool is
the reduction of internal voids and lack of fusion that are associated
with the transition region between the shoulder and the pin, as well as
the transition from the pin to the pin tip. Further, the present design
allows the use of a single tool in performing welds of varying depth
and/or width, performing welds to join workpieces having differing
thicknesses, performing welds to join workpieces having complex
curvatures, and in retracting the tool to leave a weld free of an exit
hole.

[0020]Numerous variations, modifications, alternatives, and alterations of
the various preferred embodiments, processes, and methods may be used
alone or in combination with one another as will become more readily
apparent to those with skill in the art with reference to the following
detailed description of the preferred embodiments and the accompanying
figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]Without limiting the scope of the present invention as claimed below
and referring now to the drawings and figures:

[0022]FIG. 1 shows a cross-section of a typical conventional shouldered
FSW tool, not to scale;

[0023]FIG. 2 shows a cross-section of a typical conventional shouldered
retractable pin tool, not to scale;

[0024]FIG. 3 shows a cross-section of a first workpiece and a second
workpiece as they are joined by FSW, not to scale;

[0025]FIG. 4 shows an enlarged cross-section of a portion of FIG. 3, not
to scale;

[0026]FIG. 5 shows an elevated perspective view of a first and second
workpiece being joined by FSW and the associated exit hole left by
conventional shouldered FSW tools, not to scale;

[0027]FIG. 6 shows a cross-section of a typical conventional shouldered
FSW tool and a first and second workpiece of differing thicknesses, not
to scale;

[0028]FIG. 7 shows a front elevation view of an embodiment of the tool of
the present invention, not to scale;

[0029]FIG. 8 shows a partial cross-section of a joint with the tool of
FIG. 7 joining a first and a second workpiece by FSW, not to scale;

[0030]FIG. 9 shows a first and a second workpiece configured in a lap
joint, not to scale;

[0031]FIG. 10 shows a first and a second workpiece configured in butt
joint arrangement with a third workpiece below to be joined by a lap
joint;

[0032]FIG. 11 shows a partial cross-section of a joint with an embodiment
of the tool of FIG. 7 joining a first and a second workpiece by FSW, not
to scale;

[0033]FIG. 12 shows a partial cross-section of an embodiment of the tool
of the present invention as it traverses a joint from left to right while
changing from a first penetration depth to a second penetration depth and
then is retracted from the workpieces, not to scale;

[0034]FIG. 13 shows a front elevation view of an embodiment of the tool of
FIG. 7, not to scale;

[0035]FIG. 14 shows a front elevation view of an embodiment of the tool of
FIG. 7, not to scale;

[0036]FIG. 15 shows a front elevation view of an embodiment of the tool of
FIG. 7, not to scale;

[0037]FIG. 16 shows a front elevation view of an embodiment of the tool of
FIG. 7, not to scale;

[0038]FIG. 17 shows a partial cross-section of a joint with the tool of
FIG. 7 joining a first and a second workpiece of differing thicknesses by
FSW, not to scale;

[0040]FIG. 19 shows a partial cross-section of one embodiment of the tool
of FIG. 7 traversing an undulating joint, not to scale;

[0041]FIG. 20 shows a partial cross-section of a first and a second
workpiece configured in a lap joint being welded by a typical
conventional shouldered FSW tool, not to scale;

[0042]FIG. 21 shows a partial cross-section of a first and a second
workpiece configured in a lap joint being welded by an embodiment of the
present invention, not to scale;

[0043]FIG. 22 shows a partial cross-section of a first and a second
workpiece configured in tee joint arrangement being welded by an
embodiment of the present invention, not to scale;

[0044]FIG. 23 shows a partial cross-section of one embodiment of the tool
traversing an undulating joint, not to scale;

[0045]FIG. 24 shows a partial cross-section of one embodiment of the tool
traversing an undulating joint, not to scale;

[0046]FIG. 25 shows a front elevation view of an embodiment of the tool of
the present invention, not to scale;

[0047]FIG. 26 shows a front elevation view of an embodiment of the tool of
the present invention, not to scale;

[0048]FIG. 27 shows an elevated perspective view of a first and second
workpiece being joined by a tool of the present invention, not to scale;

[0049]FIG. 28 is a photograph in transverse cross-sectional view, not to
scale, of a weld having an excessive undercut on the advancing, or left
side, and reinforcement and flash on the retreating side, or right side;
and

[0050]FIG. 29 is a photograph in transverse cross-sectional view, not to
scale, of a weld produced with the tool of the present invention having a
minimal undercut on the advancing, or left side.

DETAILED DESCRIPTION OF THE INVENTION

[0051]The non-consumable multi-section faced retractable shoulderless
variable penetration friction stir welding tool and methods of friction
stir welding of the present invention enable a significant advance in the
state of the art. The preferred embodiments of the method and apparatus
accomplish this by new and novel methods that are configured in unique
and novel ways and which demonstrate previously unavailable but preferred
and desirable capabilities. The description set forth below in connection
with the drawings is intended merely as a description of the presently
preferred embodiments of the invention, and is not intended to represent
the only form in which the present invention may be constructed or
utilized. The description sets forth the designs, functions, means, and
methods of implementing the invention in connection with the illustrated
embodiments. It is to be understood, however, that the same or equivalent
functions and features may be accomplished by different embodiments that
are also intended to be encompassed within the spirit and scope of the
invention.

[0052]The present invention includes several methods of friction stir
welding (FSW) and a non-consumable multi-section faced retractable
shoulderless variable penetration friction stir welding tool 100 for
performing the methods. The non-consumable multi-section faced
retractable shoulderless variable penetration friction stir welding tool
100 is used in joining a first workpiece 10 and a second workpiece 20
with a friction stir weld 40. The tool 100 includes a substantially
cylindrical body portion 200, a head portion 400, and a tip section 500,
each integral to the tool 100, as seen in FIG. 7. The body portion 200
has a longitudinal axis 210 about which it is rotable, a diameter 220, a
sidewall 230 substantially parallel to the longitudinal axis 210, a
proximal end 240, and a distal end 250.

[0053]The first workpiece 10 has a first thickness 12 and a top surface
14. Similarly, the second workpiece 20 has a second thickness 22 and a
top surface 24, as seen in FIG. 8. The tool 100 and methods of the
present invention work equally as well on butt joints, as seen in FIG. 5;
lap joints, as seen in FIG. 9; combination butt and lap joints, as seen
in FIG. 10; tee joints, as seen in FIG. 22; corner joints, not
illustrated but understood by one with skill in the art; as well as bead
on plate welds to alter the local characteristics of a plate due to
friction stir processing of the material with the tool.

[0054]Referring again to FIG. 7, the head portion 400 is located at the
distal end 250 of the body portion 200. The head portion 400 has a base
410 with a diameter 420 substantially equal to the diameter 220 of the
body portion 200 thereby forming a transition 300 between the body
portion 200 and the head portion 400. The head portion 400 includes a
multi-section face 440, having at least a first face section 441 nearest
the body portion and a second face section 445 nearest the tip section,
labeled in FIG. 8, that converges to the tip section 500. The tip section
500 has a diameter 510 and a center 520 wherein the center 520 is located
substantially on the longitudinal axis 210, illustrated in FIGS. 14 and
15. Referring again to FIG. 7, the head portion 400 and the tip section
500 define a height 430 from the distal-most portion of the tip section
500 to the base 410 along the longitudinal axis 210. The transition 300
may incorporate a smooth curve between the body portion 200 and the head
portion 400, but it is not required.

[0055]The substantially equal diameters 220, 420 of the body portion 200
and the head portion 400, along with the transition 300 therebetween,
establish that the present invention lacks a shoulder as is present in
prior art friction stir welding tools 90, 95, as seen in FIGS. 1 and 2.
This lack of a shoulder has numerous advantages that have long been
overlooked by those in the FSW industry.

[0056]The shoulder 91, 96 of conventional shouldered FSW tools 90 as well
as retractable pin tools 95, as seen in FIGS. 1 and 2, is the source of
many problems and confusion in FSW, which have been previously explained
in the Background of the Invention herein. In short, the present tool 100
does not require a shoulder 91, 96 to retain the plasticized material of
the FSW, contrary to the teachings of the leaders in the field. Referring
to FIG. 11, the present invention's elimination, or minimization, of any
portion of the tool 100 that contacts the top surface 14, 24 of either
workpieces 10, 20 away from the point at which the tool enters the
workpiece(s) 10, 20 has several advantages.

[0057]One such advantage is the elimination of the primary source of
overheating. Referring again to FIGS. 1-3, during FSW with prior
shouldered FSW tools, the weld 40 is repeatedly subjected to the pressure
and rotation of the tool shoulder 91, 96. As a conventional FSW tool 90,
95 traverses a joint 35 the material is first exposed to the leading edge
of the shoulder 91, 96 that is generally exerting a downward force on the
workpieces 10, 20 of several hundred pounds, often several thousand
pounds, and is rotating at several hundred RPM, while traversing the
joint rather slowly, generally less than ten inches per minute (IPM). One
with skill in the art will understand that such characteristics are
dependent on a number of factors including the material being joined and
its thickness. Taking, for example, a simple illustrative case of a
conventional tool 90, 95 traversing a joint 35 at 6 IPM and 800 RPM, it
takes 10 seconds to traverse a one inch section of the joint 35 during
which 80 revolutions of the tool 90, 95 are made, resulting in 160
exposures of weld 40 to the shoulder 91, 96 (an exposure at the leading
edge and the trailing edge for each revolution). Such repeated exposure
to the shoulder 91, 96 results in the overheating of the weld 40 and the
associated drawbacks, as previously explained. The present invention
includes a method of reducing the amount of overheating experienced by a
friction stir weld 40 by ensuring that while traversing the joint 35 with
the rotating tool 100, no portion of the tool 100, or a minimal portion
of the tool 100 substantially smaller than that of conventional FSW tool,
away from the entry penetration of the tool 100 into the workpieces 10,
20, comes in contact with the top surface 14, 24 of either workpiece 10,
20. Prior methods and apparatus have indicated that such top surface
friction heating and weld material containment were essential to FSW.

[0058]Another advantage of the present tool 100 and methods is the
reduction of internal voids and lack of fusion that are associated with
the transition region 93, labeled in FIG. 1, between the shoulder 91, 96
and the pin 92, 97 of traditional friction stir welding tools 90, 95. As
previously discussed in the Background of the Invention herein, the
transition region 93 between the shoulder 91, 96 and the pin 92, 97 is
the source of many problems in tool design and affects the
characteristics of the resulting weld. Such problems are particularly
pronounced in conventional retractable pin tools 95, illustrated in FIG.
2, because the transition region changes as the pin 97 enters the
workpieces 10, 20 or retracts from the workpieces 10, 20.

[0059]Yet another advantage of the present non-consumable multi-section
faced retractable shoulderless variable penetration tool 100 and methods
of the present invention is that the elimination of a shoulder 91 allows
the use of a single tool 100 in performing welds 40 of varying depth,
performing welds 40 to join workpieces having differing thicknesses, and
in retracting the tool 100 to leave a weld 40 free of an exit hole 80, as
seen in FIG. 5. Conventional single-piece shouldered FSW tools 90, as
seen in FIG. 1, have a fixed pin length projecting from the shoulder 91
and therefore are limited to performing welds of a single penetration
depth. The present tool 100 is designed such that the height 430 of the
head portion 400 may be (i) less than or equal to the lesser of the first
workpiece thickness 12 or the second workpiece thickness 22 such that the
entire tip section 500, head portion 400, and a portion of the body
portion 200 are in the friction stir weld 40 during welding, or
alternatively (ii) greater than or equal to the greater of the first
workpiece thickness 12 or the second workpiece thickness 22 such that the
entire tip section 500 and a portion of the head portion 400 are in the
friction stir weld 40 during welding, as seen in FIG. 11. This ability to
submerge a portion of the body portion 200 into the weld 40 permits use
of the tool 100 in creating spot welds. Additionally, the tool 100
permits the joining of a first workpiece 10 and a second workpiece 20
wherein they have unequal thicknesses 12, 14, as shown in FIG. 17.

[0060]Along with the ability to perform variable depth welds comes the
ability to vary the width of the welds. As one with skill in the art can
appreciate, the further the tool 100 of the present invention penetrates
into the joint 35 the wider the weld 40 becomes. This, along with the
ability of the present invention to be plunged into the joint 35 as it is
traversing the joint 35, permits the economical use of friction stir
welding in performing tack welds. Such tack welds are particularly useful
in holding parts in the tooling.

[0061]Additionally, one with skill in the art can appreciate that
cooperating tools may be used in creating full penetration welds in
thicker workpieces with one tool penetrating half way into the joint from
one side of the joint and a second tool penetrating half way into the
joint from the opposite side of the joint.

[0062]The shoulderless design of the present tool 100 permits the friction
stir welding of workpieces 10, 20 having significant curvature. In the
past conventional shouldered friction stir welding tools 90, 95 have not
been able to join workpieces 10, 20 having more than slight undulation
because of shoulder 91, 96 interference. As seen in FIG. 18, while
traversing down a slope the shoulder 91, 96 of conventional tools 90, 95
would lift-off, or separate from the joint 35, at either the leading edge
of the shoulder 91, 96 or the trailing edge of the shoulder 91, 96
depending on the motion control system. Alternatively, while traversing
down into a valley or up from a valley, the shoulder 91, 96 of
conventional tools 90, 95 would gouge into the joint at either the
trailing edge of the shoulder 91, 96 or the leading edge of the shoulder
91, 96 depending on the motion control system. Such lift-off and gouging
results in welds having reduced aesthetic qualities that often require
grinding of the surface and diminish the mechanical properties of the
weld.

[0063]FIG. 19 illustrates how the present tool 100 eliminates such gouging
and lift-off problems and permits the joining of workpieces 10, 20 having
aggressive curvature. Selecting a tool 100 of the present design such
that a portion of the head portion 400, and therefore a portion of the
multi-section face 440, does not penetrate the joint 35 when joining a
flat portion of the workpieces 10, 20 ensures that the body portion 200
to head portion 400 interface, or transition 300, does not gouge the
joint 35, while the multi-section face 440 remains in contact with the
joint at both the leading and trailing edges of the tool 100. In fact, it
is often preferred to perform the welding operation with a portion of the
first face section 441 within the joint, and thus the first and second
workpieces 10, 20, and a portion of the first face section 441 outside of
the workpieces 10, 20. The curve of FIG. 19 is rather gradual, yet
illustrates the point. The tool 100 of the present invention may be
utilized in joining workpieces having complimentary curves that are much
more severe. In fact, the present tool 100 may be used in configurations
where the radius R of the at least one cooperating curve is less than
approximately two times the diameter 220 of the body portion 200 and
greater than one-half the diameter 220 of the body portion 200. The
present tool 100 is illustrated in FIG. 23, with a second face opening
angle 620 of 140 degrees, traversing a curve with a radius R equal to
twice the diameter 220 of the body portion 200. Similarly, a tool 100
with a second face opening angle 620 of 70 degrees is shown in FIG. 24
traversing a curve with a radius R equal to approximately seventy-five
percent of the diameter 220 of the body portion 200.

[0064]Still further, another advantage of the present tool 100 is that it
produces wider welds 40 than those produced by conventional shouldered
friction stir welding tools 90, 95 of the same exterior diameter. FIGS.
20 and 21 illustrate that the lap joint weld width 42, being the width of
the weld 40 at the interface between the first and second workpieces 10,
20, is much greater when using a tool 100 of the present invention, as
seen in FIG. 21, than when using a conventional tool, as seen in FIG. 20.
The improved weld width 42 is a result of the relatively flat head
portion 400, when compared to prior art shouldered tools 90, 95, and
results in more bonded area between the first and second workpieces 10,
20, and thus a higher load capacity.

[0065]The relatively flat head portion 400 is also beneficial when
performing welds along tee joints, as seen in FIG. 22, and along corner
joints. The large second face opening angle 620 of the tool 100 results
in greater, and more complete, mixing of material between the first and
second workpieces 10, 20. Additionally, the backing tool 700 may be
selected to match the second face opening angle 620 of the tool 100 so
that the face 400 may be parallel to an edge of the backing tool 700 and
either touch the backing tool 700 or come into close proximity thereto,
thereby minimizing or eliminating the potential for dead zones. Further,
such a configuration has the additional benefit of aiding in the root
side fillet/chamfer formation.

[0066]Further, the design of the present invention, namely the
shoulderless transition 300 from the head portion 400 to the body portion
200, allows the weld penetration depth to change on the fly. For
instance, the tool 100 may first be plunged into the workpiece(s) 10, 20
to a first penetration depth 82 and travel for a particular distance
(left to right) before further extending, or retracting, into the
workpiece(s) 10, 20 to a second penetration depth 84, as seen in FIG. 12.
It is important to note that the present tool 100 is capable of entering
the joint 35 as it is moving along the joint 35, and need not be first
plunged to a particular depth and then traversed, as with prior tools.
For instance, the far left tool 100 of FIG. 12 could have started its
descent to the second position from the top surface rather than an
initial depth. This can be particularly advantageous in welding lap
joints, as seen in FIG. 9, and combination butt and lap joints, as seen
in FIG. 10. It is significant to note that the tool 100 of the present
invention is capable of plunging into the joint 35 as it is moving along
the joint 35, it need not be first plunged into a joint 35 and then moved
along the joint 35. Therefore, when joining the elements of FIG. 10 the
tool 100 would first enter the joint 35 between the first and second
workpieces 10, 20 to a first depth and then penetrate to a deeper depth
in the vicinity of the third workpiece 30 so as to not only join the
first workpiece 10 to the second workpiece 20 but to also join each of
them to the third workpiece 30. Such adaptability is not found in the
prior art tools.

[0067]As previously expressed, the head portion 400 includes a
multi-section face 440 that converges to the tip section 500. This
convergence may be in any manner and need not be uniform or continuous,
as seen in FIG. 13. Each section of the multi-section face 440 has a
horizontal projection component, namely a first section horizontal
projection component 442 and a second section horizontal projection
component 446, as seen in FIG. 25. The first section horizontal
projection component 442 represents the magnitude of the portion of the
first face section 441 that is orthogonal to the longitudinal axis 210.
Similarly, the second section horizontal projection component 446
represents the magnitude of the portion of the second face section 446
that is orthogonal to the longitudinal axis 210. The magnitude of the
first section horizontal projection component 442 is less than fifteen
percent of the body portion diameter 220.

[0068]In one embodiment, the second face section 445 forms a substantially
frustoconical shape with the second face section 445 converging to the
tip section 500 at a second face opening angle 620, as seen in FIG. 25.
The second face opening angle 620 may be virtually any angle but the
range of between approximately 70 degrees and approximately 160 degrees,
illustrated in FIG. 15, has been found to be effective, with the range of
approximately 100 degrees and 140 degrees even more preferred. A second
face opening angle 620 of 90 degrees is illustrated in FIG. 14. The
relatively flat head portion 400 and tip section 500 of the present
invention also flies in the face of traditional FSW teachings.

[0069]In a further embodiment, the first face section 610 converges to the
second face section 620 at a first face opening angle 610 to form a
substantially frustoconical shape, as seen in FIG. 25. The first face
opening angle 610 may be virtually any angle. For example, in one
embodiment the first face opening angle is approximately 180 degrees,
making the first face section 441 substantially orthogonal to the
longitudinal axis 210, as seen in FIG. 15. In this embodiment the
magnitude of the first section horizontal projection component 442 is
less than approximately eight percent of the body portion diameter 220,
and has worked effectively during experimentation at less than
approximately five percent of the body portion diameter 220. In fact, a
magnitude of the first section horizontal projection component 442 of
between approximately two percent to approximately four percent of the
body portion diameter 220 has been found to be particularly effective
when joining titanium workpieces, while a magnitude of between
approximately four percent to approximately eight percent of the body
portion diameter 220 has been found to be particularly effective when
joining steel workpieces. In an alternative embodiment the first face
opening angle 610 is between approximately 40 degrees and 120 degrees. In
further embodiments the first face section 441 is curved. One particular
curved embodiment, shown in FIG. 26, has a radius of curvature R of less
than fifteen percent of the body portion diameter 220. Regardless of the
first face opening angle 610 or shape, the magnitude of the first section
horizontal projection component 442 is less than fifteen percent of the
body portion diameter 220, and is generally selected based upon the
material of the workpieces.

[0070]The purpose of the first face 441 is to minimize the undercut, or
under fill, on the advancing side of the tool 100. The advancing side of
the tool 100 is the side of the tool 100 where the local direction of the
multi-section face 440 due to tool rotation and the direction that the
tool 100 is traversing T are in the same direction. Therefore, the
advancing side of the tool 100 in FIG. 27 is the left side of the tool.
Alternatively, the retreating side of the tool 100 is the side of the
tool 100 where the local direction of the multi-section face 440 due to
tool rotation and the direction the tool 100 is traversing T are in the
opposite direction. Therefore, the retreating side of the tool 100 is
FIG. 27 is the right side of the tool. Advancing side undercut, or under
fill, is caused when plasticized weld material flows around the advancing
side of the tool 100 and ends up on the retreating side of the weld in
the form of increased local weld thickness, also referred to as positive
reinforcement, or in the worst case loosely attached flash, thus leaving
an area of reduced weld thickness, or undercut and under fill, on the
advancing side of the weld, as seen in FIG. 28, a cross-sectional photo
of a friction stir weld having undercut on the advancing side. Advancing
side undercut conditions are particularly prevalent when the second face
opening angle 620 is less than 140 degrees. A photograph in transverse
cross-sectional view of a weld produced with the tool 100 of the present
invention is shown in FIG. 29 having a minimal undercut on the advancing,
or left side.

[0071]The first face 441 minimizes the undercut by increasing the amount
of plasticized weld material that is dragged around the tool 100 from the
retreating side to the advancing side, thereby infilling the undercut
area. The size and configuration of the first face section 441 play a
direct role in reducing the undercut and on tool 100 performance. The
larger the first face section 441 the more plasticized weld material is
transferred to the advancing side, yet a large first face section 441
reduces the robustness of the tool 100. Therefore, referring again to
FIG. 25, the present invention recognizes this balancing act and has
related it to the first section horizontal projection component 442. It
is preferential to have the magnitude of the first section horizontal
projection component 442 less than fifteen percent of the body portion
diameter 220. Further, the magnitude of the first section horizontal
projection component 442 preferably selected depending on the material of
the workpieces. For example, when joining titanium workpieces, which do
not transfer the heat created during FSW particularly well, the magnitude
of the first section horizontal projection component 442 may be
relatively small because the plasticized titanium adheres well to the
first face section 441. Conversely, when joining steel workpieces, which
transfer the heat created during FSW particularly well, the magnitude of
the first section horizontal projection component 442 must be larger
because the plasticized steel does not adhere well to the first face
section 441. Therefore, the magnitude of first section horizontal
projection component 442 necessary when joining titanium is roughly fifty
percent of the magnitude necessary when joining steel. For example, a
first section horizontal projection component 442 having a magnitude of
eight percent, or less, of the body portion diameter 220 has been found
to work particularly well when joining titanium workpieces, whereas a
first section horizontal projection component 442 having a magnitude of
twelve percent, or less, of the body portion diameter 220 has been found
to work particularly well when joining steel workpieces. In particular,
in the embodiments wherein the first face opening angle 610 is between
approximately 40 degrees and approximately 120 degrees a magnitude of the
first section horizontal projection component 442 of between
approximately two percent to approximately four percent of the body
portion diameter 220 has been found to be particularly effective when
joining titanium workpieces, while a magnitude of between approximately
four percent to approximately eight percent of the body portion diameter
220 has been found to be particularly effective when joining steel
workpieces. Further, in the embodiments wherein the first face section
600 is curved experimentation has shown that a magnitude of the first
section horizontal projection component 442 of between approximately
three percent to approximately six percent of the body portion diameter
220 has been found to be particularly effective when joining titanium
workpieces, while a magnitude of between approximately six percent to
approximately twelve percent of the body portion diameter 220 has been
found to be particularly effective when joining steel workpieces.

[0072]In one embodiment the tip section 500 is a flat shape 540, as seen
in FIGS. 7 and 15. Alternatively the tip section 500 may be a curved
shape 530, as seen in FIGS. 14 and 11. Still further, the tip section 500
may by pyramidal in shape, or virtually any other shape imaginable. Since
the head portion 400 converges to the tip section 500 there will always
be tip section diameter 510 at the interface between the tip section 500
and head portion 400, as seen in FIGS. 14 and 15. It is at the tip
section diameter 510 that the tip section 500 transitions to the head
portion 400. In one embodiment the tip diameter 510 is less than
approximately forty percent of the body portion diameter 220 or the head
portion diameter 420. Such an aggressive convergence is unlike prior FSW
tools. In some embodiments the tip section 500 continues to converge at
the same angle as the head portion 400 and is therefore indistinguishable
from the head portion 400, as in the case of a simple cone seen in FIG.
24.

[0073]The multi-section face 440 of the head portion 400 and the sidewall
230 of the body portion 230 may be substantially smooth or contain
friction and/or plunge control features. For instance, in one embodiment
the multi-section face 440 of the head portion 400 is formed with at
least one recess 450, as seen in FIG. 16, to aid in heat generation;
stirring of the weld 40; reduction of surface flash formation; and
improved stability of the tool 100 during the plunge. Alternatively, the
multi-section face 440 may include projections extending from the
multi-section face 440 such as threads or stipples, as disclosed in the
prior art.

[0074]The present tool 100 also eliminates the points of high stress
concentration present in conventional prior art shouldered tools 90, 95.
Typically the pin 92, 97 of conventional prior art shouldered tools 90,
95 is approximately one-third the diameter of the overall tool diameter,
as seen in FIGS. 1 and 2. This change in diameter occurs at the shoulder
91, 96 and is a point of particularly high stress in the pin 92, 97.
Obviously, the present design seen in FIG. 11 does not contain such
points of high stress concentration. Further, the useful life of a tool
100 of the present design is significantly greater than that of
conventional prior art shouldered tools 90, 95.

[0075]While the disclosure herein refers generally to a first workpiece 10
and a second workpiece 20, the present invention may be used in joining
more than just two workpieces or in the repair of a single workpiece. For
example, the tool and methods of the present invention may be used in
friction stir processing of a single workpiece to improve its properties.

[0076]Numerous alterations, modifications, and variations of the preferred
embodiments disclosed herein will be apparent to those skilled in the art
and they are all anticipated and contemplated to be within the spirit and
scope of the instant invention. For example, although specific
embodiments have been described in detail, those with skill in the art
will understand that the preceding embodiments and variations can be
modified to incorporate various types of substitute and or additional or
alternative materials, relative arrangement of elements, and dimensional
configurations. Accordingly, even though only few variations of the
present invention are described herein, it is to be understood that the
practice of such additional modifications and variations and the
equivalents thereof, are within the spirit and scope of the invention as
defined in the following claims. The corresponding structures, materials,
acts, and equivalents of all means or step plus function elements in the
claims below are intended to include any structure, material, or acts for
performing the functions in combination with other claimed elements as
specifically claimed.